In gas quenching, parts are sealed in a vacuum chamber before being blasted with gases. Figure 29. Cutting tools use high-speed tool steels that have special alloy compositions. Quenching metal is one of the critical stages in the heat treatment of a metal part because it’s during that process that added hardness is locked in. Marquenching is used primarily to minimise distortion and eliminate cracking. Grossmann's method of measuring hardenability uses a number of cylindrical steel bars of different diameters hardened in a given quenching medium (30). Additives are used to improve functionality properties such as increased wetting capacity, increased cooling capacity, vapor film dissipation properties, and improved washing properties, as well as greater oxidation stability for increased service life. Normally, size and shape changes vary in the direction of the ring axis, which causes changes in the angle of ring raceways. The most commonly used quenchants today are quenching oils, polymer quenchants, salt baths, and certain gases. Gas insulated high voltage lines are now in operation using these gas mixtures. (a) Jominy hardenability curve and (b) cross-sectional hardness curve for a 4-inch diameter of a quenched round bar of the same steel and steel chemistry. When hardness can be sacrificed, mineral oils are often used. Figure 2. The gas sublimes at about − 64°C and it may be used at temperatures up to about 150°C. Here, an entirely new set of conditions exists. Note: The value of ‘r/R’ refers to the radius of the unhardened core ‘r’ to the value of the total radius of the bar ‘R’. The cooling capacity depends on choice of the gas, its flow rate and pressure. (Compare to Figure 18.) and as an arc-, One objective in conducting the Jominy end-quench test is to determine the adequacy of using a particular quenchant to harden a steel with a specific hardenability. Illustration of Jominy H-bands for AISI 4130 and 4140 steels. Keeping in mind that these rings were austenitized at 870 °C, it is clear that more carbon was dissolved and therefore a larger volume change is expected. Hot oils—which are kept at higher temperatures—cool metal surfaces, but not so quickly that a part’s core temperature and surface temperature differ too widely. at a surge frequency of 5–100 kHz or more, depending upon the interrupting circuit constants L and C. They may exist in the system for a slightly longer duration of one half to one-and-a-half cycles (10–30 ms for a 50 Hz system), i.e. The terms ‘Rapid quenching oils’ (applied at about 60 °C), ‘General quenching oils’ (applied at about 60–130 °C), and ‘Hot quenching oils’ (applied at about 120–200 °C) are used. In this study, medium carbon steel was used by virtue of its Quenching is carried out for altering and achieving the desired mechanical properties of indust Table 29.10 summarises the mean carbon content limits for water, brine or caustic quenching of some steels.33,35, Table 29.10. When this occurs, it causes an arc between the contacts leading to such surges. Reproduced from Asimow, M.; Craig, W. F.; Grossmann, M. A. In circuit-breakers this problem is overcome by careful selection of materials (e.g. Austenitize, Quench… In steels of high alloy content, it is the usual practice to control the carbon of the case to a somewhat lower level than in low alloy and plain carbon steels. These oils cannot be used for shallow- hardening steels. In general, any steel part or grade of steel responding to oil quenching can be marquenched to provide similar physical properties. Surges that may appear on an LT system as a result of transference from the HV side of a transformer (Section 18.5.2) are different and not related to switching. High-alloy tool steels and jet engine turbines are common examples of parts often quenched in gas. Water is one of the most efficient quenching media where maximum hardness is required, but it is liable to cause distortion and cracking of the work piece. Cooling curves for different quenchants. Also helium can be used but then gas recycling is necessary to minimize consumption and make the process cost effective. The cooling rates of mineral quenching oils are generally highest at about 550–650 °C and are low at temperatures close to those at which martensite forms in low alloy steels which is beneficial regarding distortion. Gas quenching is used primarily in connection with hardening in vacuum furnaces, but it can also be used with the gas applied by nozzles or fans at a high flow rate and a relatively low pressure. Silicon is frequently used with manganese to obtain a strong, tough steel. These kinds of alloy are used in many different Air is a popular quenching media used to cool metals for quenching. The rings quenched in a gas nozzle field show the maximum volume change. Consequently cracking and distortion of the object are liable to occur after quenching. Details of such surges and causes of their generation have been given in Chapter 17. In general, any steel part or grade of steel responding to oil quenching can be marquenched to provide similar physical properties. The curve most representative of the H-value of the quenching medium to be used is then identified. ; Hoffmann, F.; Zoch, H. -W. Distortion of Rings due to Inhomogeneous Temperature Distributions. Table 13. Some figures relating to the electric strength of sulphur hexafluoride and mixtures of this gas with nitrogen are given in Table 7.7. In addition, workers must take special precautions when using caustic materials because they’re harmful when inhaled or exposed to skin and eyes. The most common polymer quenchants are based on acrylates and glycols. These hardness values are used with Figure 28. It is assumed that equivalent cooling rates at two positions in a steel bar will exhibit the same hardness. SF6 is an inorganic compound; it is a colorless, odorless, nontoxic, and nonflammable gas. Sulphur hexafluoride is an electronegative gas which has come into wide use as a dielectric (in X-ray equipment, in waveguides, coaxial cables, transformers, etc.) Lamont developed a methodology based on the Jominy hardenability test and a fractionally hardened round bar (75,76). The Grossmann H-Factor is defined as the ratio of the heat transfer coefficient at the surface of the steel specimen divided by twice the heat conductivity of the steel. For an austenitizing temperature of 850 °C, the volume change of the ball bearing steel SAE 52100 amounts to 1.7% after a correct hardening (35). Steel is quenched in water or brine for the most rapid cooling, in oil for some alloy steels, and in air for certain higher alloy steels. a blacksmith plunges an axe or hatchet into cold water to temper it—for it is this that gives strength to the iron . The martensite of quenched tool steel is exceedingly brittle and highly stressed. High-alloy steels, which are much more hardenable, are best quenched in less severe media. Some illustrative values are provided in Table 29.9.9,33 Typically, the greater the quench severity, the greater the propensity of a given quenching medium to cause increased distortion or cracking. Reproduced from Surm, H.; Frerichs, F.; Lübben, Th. ∗Both types of gas flow direction: top/bottom and bottom/top. In addition, the distortion behavior of the outer raceway is also characterized by a change in amplitude of the second order. There is, however, a major factor unique to tool steels that must always be kept in mind—the effect of undissolved carbide. In Figure 26(f), the point where the line intersects the Jominy distance of ¾ inch indicates that the Grossmann H-value is 2.0 (76). The heat treatment expertise and equipment available at Paulo ensure your parts meet specifications and remain strong and useful for longer. With the exception of oil quenching, no major difference can be identified between the examined quenching technologies. Selection of a proper quenchant for a particular case is determined mainly by the hardenability of the steel, the dimensions of the workpiece, and the desired properties. Mat.-wiss. However, using water can lead to metal cracking or becoming distorted. Shift the Ms and Mf points for martensite transformation to higher or lower temperatures, thereby changing the amount of austenite retained on quenching or after refrigeration treatment. The various microstructures developed after quenching the carbon steel in selected quenching media are presented in Table 4 and Micrographs 1-7. have been noticed in certain switching circuits, with a front time as low as 0.2 μs. When hot oil quenching is used for carburised steels, lower bainite, which exhibits properties somewhat similar to those of martensite, is formed. However, quenching directly from the carburizing temperature may leave retained austenite; when high alloy content steels are treated to minimize the retained austenite this technique is not suitable. Brine (salt water) : Brine is a more severe quench medium than water.Unfortunately it tends to accelerate corrosion problems unless completely removed.Sodium or potasium hydroxide can be used when very severe quenching is desiredand one wishes to obtain good hardness in low carbon steels Different quenching media have different degrees of severity. Table 3. Each figure shown in Figure 26(a–f) represents a different ratio of r/R. Variation of the quenching cycle for a quenchant can be effected by varying its temperature or degree of agitation. Figures 21 and 22 illustrate the Jominy H-bands for AISI 1020 and 1040 carbon steels and AISI 4130 and 4140 low-alloy steels, respectively (38). Therefore, it is important to validate these approximations experimentally. Our polymer quenching hardening heat treatment can be the solution for jobs that do not respond properly to oil quenching. Quenching cycles. 2012, 43 (1–2), 29–36. The best steels for austempering are plain carbon, Cr and Mo alloy steels.34. oils compromise the hardness of steel so water is the best quenching media for steel. Helium and argon are also used in gas quenching. High-alloy parts with intricate designs quench well in hot oils, as the method reduces the risk of warping and cracking associated with differences in surface and core temperatures. Fast interrupting switches [111] rely on sulfur hexafluoride (SF6) as a quenching medium resulting in an increased interrupting capability of the circuit breaker. However, they are not really suitable for use in protective atmosphere furnaces, where the quenching bath is integrated in the furnace, as the water vapor emitted would spoil the furnace atmosphere. Rushman developed an approach to predict the as-quenched hardness of a steel bar of known hardenability by using the Jominy diagram for the steel of interest, a two-diameter test bar shown in Figure 27, and a modified Grossmann chart shown in Figure 28 (74). Otherwise they are difficult to … at a surge frequency of 5–100 kHz or more, depending upon the closing circuit constants L and C. They may exist in the system for a very short duration of much less than even one half of a cycle, i.e. L.C.F. Additives are used to reduce foam, inhibit corrosion, and reduce bacteria growth. SF6 has an extremely great influence on global warming and therefore its use should be minimized or entirely avoided. Some are unclear. Let us know how we can help with your next heat treatment job, and view the guide below to learn more about the role of quenching in heat treating. Addition of nitrogen often improves some of the characteristics, while at the same time reducing the overall cost. Table 6. To identify a quenching medium and its condition, Grossmann introduced the quenching intensity (severity) value H. Table 3 provides a summary of Grossmann H-values for different quenching media and different quenching conditions (52). Too much time between the quenching and the tempering of the heat-treated parts. The quenching operation is one of the most critical, and usually the least controllable, part of the heat treatment process. (Refer to Section 17.7.3). Quenching and tempering provides steel with high strength and ductility. Salt bath quenchants are molten salts, normally comprising a combination of nitrite and nitrate salts, which melt at about 150–180 °C. Water and oil are the most commonly used as quenching media in heat treatment processes to harden steel alloy. We also deal briefly with the constructional features and application of such breakers. Canale, ... G.E. Silicon is frequently used with manganese to obtain a strong, tough steel. I almost never use water other than for low carbon or non-hardenable material. Modern heat treatment processes are con- stantly making new demands on quenching fluids. Chart for predicting approximate as-quenched cross-sectional hardness from Jominy curve data. This process of rapidly cooling of steel is referred to as quenching and the media in which the steel is quenched is called quenchant [17]. From the Jominy distance, the hardness is determined from the Jominy curve. The series connection of an FIS with a fault-current limiter either reduces the interrupting conditions or makes the substitution of SF6 with a less effective quenching gas possible. This study has shown the oils under investigation to contain mainly fatty acids in their chemical composition . T Series: Tungsten High-Speed Tool Steels, these are quenched in air, oil or molten salt. Cooling capacity, expressed as the Grossmann H-factor, for different quenchants. Nitrogen is the most popular gas quenchant due to its relatively low atomic mass, wide availability and low cost. up to the closure of the switch only. In this chapter we discuss the types of insulating and quenching mediums, their switching characteristics and merits and demerits of their use. The value of r/R is ‘0’ at the center and ‘1.0’ at the surface. The value of r/R = 1.4/2.0 = 0.7, which corresponds to Figure 26(h). Using Figure 26(h) for the depth of hardening of 0.6 inch, the intersection of the distance from the quenched end (9/16) and 4 inch (diameter of the round bar) corresponds to approximately H = 1.5. For the same composition, the Jominy H-bands for as-cast and wrought steel will be the same. Reproduced from Holm, T.; Olsson, P.; Troell, E. AJ Pearmain BSc(Eng), PhD, MIEE, CEng, A Haddad Ing.d'Etat, PhD, in, Electrical Engineer's Reference Book (Sixteenth Edition), Sulphur hexafluoride is an electronegative gas which has come into wide use as a dielectric (in X-ray equipment, in waveguides, coaxial cables, transformers, etc.) Through-hardening is determined from Figure 26(a), where the fractional depth of hardening r/R = 0.0. It is necessary, therefore, to warm the steel below the critical range in order to relieve stresses and to allow the arrested reaction of cementite precipitation to take pla… Hardening of steel is obtained by a suitable quench from within or above the critical range. Polymer quenching is particularly useful for quenching high integrity alloy steel forgings that require impact testing. Although the gas is considered to be non-toxic, non-flammable and chemically inert, under the influence of arcs or high-voltage discharges, there may be some decomposition with consequent attack on certain insulating materials and metals, and more importantly some recent environmental concerns. Quenching is a heat treatment process that involves heating steel to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hyper-eutectoid steel), preserving the heat for a period of time to allow for complete or partial austenitization of the steel, then cooling it off rapidly at a rate above the critical cooling rate, to a temperature below Ms for martensite(or bainite) transformation (or isothermal near Ms)… The formation of retained austenite is a significant problem with austempering processes. If an aluminum alloy is slowly cooled from an elevated temperature, alloying elements precipitate and dif-fuse from solid solution to concentrate at the grain boundaries, small voids, on undissolved particles, at disloca-tions, and other imperfections in the aluminum lattice as shown in Fig. Quenched and tempered carbon steel for machine structural use (e.g., S45C class) had been applied for-merly as a steel material for construction machinery parts and other applications with similar requirements. Reproduced from Rushman, W. F. How to Determine Quench Severity of Oil and Salt Baths. Asimow et al. Quenching is a stage of material processing through which a metal is quickly brought down to room temperature from a high temperature by rapid cooling. Thus, the maximum bar diameter of the steel represented by the Jominy curve in Figure 25(a) that will be through-hardened when quenched in a petroleum oil with an H-value of 0.35 is 1.5 inch. Cooling proceeds through three separate stages during a quenching operation. The different quenching technologies lead to different alterations of the mean angle. In some cases, it is desirable to quench a steel so that complete through-hardening (r/R = 0) does not occur. This result arises from a dependency on ring position in the batch. The Dcrit value is valid only for the quenching medium and conditions used to determine this value. The test bar is austenitized with the production load of parts, typically in the same basket with the parts being heat treated, and then quenched. Alloy steels are generally more adaptable to marquenching. The theory of arc interruption is the same for all switching devices. The two-diameter test bar is used to evaluate the quenchant under actual production conditions. The harden-ability of a steel-alloy is dependent on its carbon or other alloys content. Cooling capacity can be increased by quenching in a separate cold chamber instead of in the furnace. Because oil is flammable, workers must know the flashpoint of the oil in use as well as the load weight and surface area of the products in the workload to avoid fires during quenching. Manganese, silicon, nickel, chromium, molybdenum, vanadium, aluminum and boron are commonly present in these steels to enhance the properties obtainable after quenching and tempering. Boegehold also developed an approach that involves a series of cures that interrelates bar diameter, quench severity (H-value), and Jominy data for the prediction of round-bar hardness at the surface, mid-radius (1/2R), and center (R) (78,79). Impact energy to failure was found using a Charpy impact tester. Although these data have been published in numerous reference texts for many years, they are of relatively limited quantitative value. To improve weldability, they are welded using the furnace induction method. For the outer raceways of the rings, the results are presented in Figure 37. After austenitizing and quenching, the two-diameter bar is sectioned, and the hardnesses at the center are determined at three points: at the center of each diameter and at the ‘common point’. As discussed previously, steel hardenability is dependent on composition, which varies within specified limits for each steel grade. Next to water and aqueous solutions, the most widely used quenching medium is oil. ; Hoffmann, F.; Zoch, H. -W. Distortion of Rings due to Inhomogeneous Temperature Distributions. The temperatures are the same as those given for full annealing. On the other end of the spectrum, some salt mixtures have high melting points and working ranges and can be used to heat parts. Therefore, many steels are also available as H-grade, which possesses tighter compositional control and the control is specified by the maximum and minimum allowable hardenability. The resulting microstructure of these rings should consist of an appreciable portion of bainite and/or pearlite in the martensitic matrix, which was confirmed in hardness values between 40 and 55 HRC in the quench-hardened state. Cooling curves for oils and polymers may vary in a great range depending on, e.g., type, temperature, concentration, and agitation. These figures illustrate the Lamont H-value correlation of Jominy end-quench distance and round-bar diameter at different locations in the round bar. A higher pressure or use of helium instead of nitrogen during gas quenching will result in faster cooling. The quenchant can be applied by different methods, e.g., dipping the workpiece or load in a bath or applying the quenchant by spraying as for induction hardening. A guiding principle is that a quenchant must have sufficient cooling capacity for the steel to achieve the desired hardness but must not quench so rapidly that quench cracks or undesirable distortion occur. 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